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Deprecated since version 2.6: The compiler package has been removed in Python 3.0.

The Python compiler package is a tool for analyzing Python source code and
generating Python bytecode. The compiler contains libraries to generate an
abstract syntax tree from Python source code and to generate Python
bytecode from the tree.

The compiler package is a Python source to bytecode translator written in
Python. It uses the built-in parser and standard parser module to
generated a concrete syntax tree. This tree is used to generate an abstract
syntax tree (AST) and then Python bytecode.

The full functionality of the package duplicates the built-in compiler provided
with the Python interpreter. It is intended to match its behavior almost
exactly. Why implement another compiler that does the same thing? The package
is useful for a variety of purposes. It can be modified more easily than the
built-in compiler. The AST it generates is useful for analyzing Python source
code.

This chapter explains how the various components of the compiler package
work. It blends reference material with a tutorial.

Returns an abstract syntax tree for the Python source code in buf. The
function raises SyntaxError if there is an error in the source code. The
return value is a compiler.ast.Module instance that contains the tree.

Compile the string source, a Python module, statement or expression, into a
code object that can be executed by the exec statement or eval(). This
function is a replacement for the built-in compile() function.

The filename will be used for run-time error messages.

The mode must be ‘exec’ to compile a module, ‘single’ to compile a single
(interactive) statement, or ‘eval’ to compile an expression.

The flags and dont_inherit arguments affect future-related statements, but
are not supported yet.

There are some problems with the error checking of the compiler package. The
interpreter detects syntax errors in two distinct phases. One set of errors is
detected by the interpreter’s parser, the other set by the compiler. The
compiler package relies on the interpreter’s parser, so it get the first phases
of error checking for free. It implements the second phase itself, and that
implementation is incomplete. For example, the compiler package does not raise
an error if a name appears more than once in an argument list: deff(x,x):...

The compiler.ast module defines an abstract syntax for Python. In the
abstract syntax tree, each node represents a syntactic construct. The root of
the tree is Module object.

The abstract syntax offers a higher level interface to parsed Python source
code. The parser module and the compiler written in C for the Python
interpreter use a concrete syntax tree. The concrete syntax is tied closely to
the grammar description used for the Python parser. Instead of a single node
for a construct, there are often several levels of nested nodes that are
introduced by Python’s precedence rules.

The abstract syntax tree is created by the compiler.transformer module.
The transformer relies on the built-in Python parser to generate a concrete
syntax tree. It generates an abstract syntax tree from the concrete tree.

The transformer module was created by Greg Stein and Bill Tutt for an
experimental Python-to-C compiler. The current version contains a number of
modifications and improvements, but the basic form of the abstract syntax and of
the transformer are due to Stein and Tutt.

The compiler.ast module is generated from a text file that describes each
node type and its elements. Each node type is represented as a class that
inherits from the abstract base class compiler.ast.Node and defines a
set of named attributes for child nodes.

The Node instances are created automatically by the parser generator.
The recommended interface for specific Node instances is to use the
public attributes to access child nodes. A public attribute may be bound to a
single node or to a sequence of nodes, depending on the Node type. For
example, the bases attribute of the Class node, is bound to a
list of base class nodes, and the doc attribute is bound to a single
node.

Each Node instance has a lineno attribute which may be
None. XXX Not sure what the rules are for which nodes will have a useful
lineno.

Returns a flattened list of the child nodes and objects in the order they
occur. Specifically, the order of the nodes is the order in which they
appear in the Python grammar. Not all of the children are Node
instances. The names of functions and classes, for example, are plain
strings.

Returns a flattened list of the child nodes in the order they occur. This
method is like getChildren(), except that it only returns those
children that are Node instances.

Two examples illustrate the general structure of Node classes. The
while statement is defined by the following grammar production:

while_stmt: "while" expression ":" suite
["else" ":" suite]

The While node has three attributes: test, body, and
else_. (If the natural name for an attribute is also a Python reserved
word, it can’t be used as an attribute name. An underscore is appended to the
word to make it a legal identifier, hence else_ instead of
else.)

The if statement is more complicated because it can include several
tests.

The If node only defines two attributes: tests and
else_. The tests attribute is a sequence of test expression,
consequent body pairs. There is one pair for each if/elif
clause. The first element of the pair is the test expression. The second
elements is a Stmt node that contains the code to execute if the test
is true.

The getChildren() method of If returns a flat list of child
nodes. If there are three if/elif clauses and no
else clause, then getChildren() will return a list of six
elements: the first test expression, the first Stmt, the second text
expression, etc.

The following table lists each of the Node subclasses defined in
compiler.ast and each of the public attributes available on their
instances. The values of most of the attributes are themselves Node
instances or sequences of instances. When the value is something other than an
instance, the type is noted in the comment. The attributes are listed in the
order in which they are returned by getChildren() and
getChildNodes().

There is a collection of nodes used to represent assignments. Each assignment
statement in the source code becomes a single Assign node in the AST.
The nodes attribute is a list that contains a node for each assignment
target. This is necessary because assignment can be chained, e.g. a=b=2. Each Node in the list will be one of the following classes:
AssAttr, AssList, AssName, or AssTuple.

Each target assignment node will describe the kind of object being assigned to:
AssName for a simple name, e.g. a=1. AssAttr for an
attribute assigned, e.g. a.x=1. AssList and AssTuple for
list and tuple expansion respectively, e.g. a,b,c=a_tuple.

The target assignment nodes also have a flags attribute that indicates
whether the node is being used for assignment or in a delete statement. The
AssName is also used to represent a delete statement, e.g. delx.

When an expression contains several attribute references, an assignment or
delete statement will contain only one AssAttr node – for the final
attribute reference. The other attribute references will be represented as
Getattr nodes in the expr attribute of the AssAttr
instance.

This section shows several simple examples of ASTs for Python source code. The
examples demonstrate how to use the parse() function, what the repr of an
AST looks like, and how to access attributes of an AST node.

The first module defines a single function. Assume it is stored in
/tmp/doublelib.py.

"""This is an example module.This is the docstring."""defdouble(x):"Return twice the argument"returnx*2

In the interactive interpreter session below, I have reformatted the long AST
reprs for readability. The AST reprs use unqualified class names. If you want
to create an instance from a repr, you must import the class names from the
compiler.ast module.

The visitor pattern is ... The compiler package uses a variant on the
visitor pattern that takes advantage of Python’s introspection features to
eliminate the need for much of the visitor’s infrastructure.

The classes being visited do not need to be programmed to accept visitors. The
visitor need only define visit methods for classes it is specifically interested
in; a default visit method can handle the rest.

The ASTVisitor is responsible for walking over the tree in the correct
order. A walk begins with a call to preorder(). For each node, it checks
the visitor argument to preorder() for a method named ‘visitNodeType,’
where NodeType is the name of the node’s class, e.g. for a While node a
visitWhile() would be called. If the method exists, it is called with the
node as its first argument.

The visitor method for a particular node type can control how child nodes are
visited during the walk. The ASTVisitor modifies the visitor argument
by adding a visit method to the visitor; this method can be used to visit a
particular child node. If no visitor is found for a particular node type, the
default() method is called.

The code generator is a visitor that emits bytecodes. Each visit method can
call the emit() method to emit a new bytecode. The basic code generator
is specialized for modules, classes, and functions. An assembler converts that
emitted instructions to the low-level bytecode format. It handles things like
generation of constant lists of code objects and calculation of jump offsets.